dissect: implement the same logic as gpt-auto-generator

dissect: implement the same logic as gpt-auto-generator

gpt-auto-generator does three checks: 1. whether the directory doesn't have
files, 2. whether it's not in fstab, and 3. whether it is not a mount point.
For dissect logic, 3. is not relevant, and it ignores 2.
But the check whether files exists was done only partially: it was done
for /efi, but not for /boot.

Two changes are made:
- the check whether /boot is empty is now done.
- for ESP, /boot is used in preference to /efi, if not used for XBOOTLDR.

With those changes, the logic in dissect matches what gpt-auto-generator does.
There are the two intentional differences described in the first paragraph,